A privacy securing film is also referred to as a light control film or the like. The demand for the privacy securing film has increased according to a tendency in taking serious view on each individual private life. Accordingly, many studies on the privacy securing film have been made, mainly for application to a window blind and the like. For example, U.S. Pat. No. 2,053,175 to E. Astima discloses a method of manufacturing a louver film by alternately laminating transparent materials and opaque materials and cutting the laminate vertically. In addition, U.S. Pat. No. 2,689,387 to Carr, W. P. et al. discloses a plastic blind, which is manufactured by alternately arranging transparent films and optical shielding films, bonding them to each other and then cutting the laminate vertically. Further, U.S. Pat. No. 3,524,789 to Olsen, F. O. et al. discloses a method of manufacturing a privacy securing film by slicing a stacked billet. Recently, with the development of display devices such as LCDs, an optically securing film for protecting individual privacy, i.e., for not showing the contents displayed on a display device to others at a certain angle besides, has been increasingly demanded. However, a conventional louver film like disclosed in afore-mentioned articles produces a ghost image, due to a variation in the refraction index of a plastic part at the interface between a transparent portion and an opaque portion, thereby causing a fatigue or tiredness to users giving their attention to a display. Therefore, recent researches on privacy securing films have been focused on reduction in the creation of a ghost image. To reduce the creation of the ghost image, International Publication No. WO 92/11549 discloses a light control film employing a multi-layered louver that comprises high optical absorption layers containing a relatively high content of carbon black and low optical absorption layers containing a relatively low content of carbon black. However, the privacy securing films in the aforementioned documents employ complicated louver structures, which lead to complicated manufacturing processes and high production costs.
In order to overcome the problems of the conventional privacy securing films including above mentioned inventions, we had invented and granted to a Korean patent No. 10-500477 regarding a privacy securing film having a louver film, wherein the louver film is manufactured through a process including the steps of i) alternately laminating and thermo-compressing transparent film layers and opaque adhesive ink layers, and ii) cutting the thermo-compressed, laminated film perpendicularly to or at a certain angle with respect to the surface thereof such that the thickness of the louver film is in a range of 2 to 30 times the thickness of the transparent film, which can be manufactured through a simplified and economically efficient process, while reducing eyestrain through a decrease in surface reflection and also minimizing the creation of a ghost image. The resin contained in the opaque adhesive ink is diffused into some of the adjacent transparent film layers during thermo compression to combine the opaque adhesive ink layers with the transparent film layers, which also allows a part of the pigment of high light absorptivity to be diffused into the transparent film layers along with the resin. As for the reason of the reduction in the creation of a ghost image, the inventor presumed that some part of the opaque adhesive ink is transferred into the transparent film layers during the thermo-compression, and thus, changes in physical and chemical properties such as morphology and reflectivity are caused at the interface between the transparent film layer and the opaque adhesive ink layer, thereby reducing the creation of the ghost image in the privacy securing film.